Sword and Fist has good ideas, but was originally marred by serious rules problems (the sheer quantity of erratta is impressive). Even with the fixes, some of it doesn't seem quite right.
Defenders of the Faith has a few balance issues, and is generally not very imaginative. It suffers from power escalation in some cases.
Tome and Blood has really cool ideas, executed not too badly. Not particularly solid though. The second best in my opinion.
Song and Silence is very solid mechanically, and very well balanced, and the traps mechanics stuff is badly needed, but it suffers from a lack of imagination. It all seems very pedestrian, other than the Fang of Lolth, which is pretty much unuseable in most games.
Masters of the Wild seems to have the best features of both Tome and Blood and Song and Silence. I don't see any glaring balance issues, it's stuffed with cool imaginative content, and the erratta needed seems pretty minimal.
My vote went to MotW.
--Seule